The facts which have been discussed in the preceding pages offer a sufficient explanation of the causes which operate in those deliming processes which depend on the simple neutralisation of the alkaline matters present in the hide, and of the swelling by means of acid which forms a step in the manufacture of many sorts of sole-leather, but they by no means fully elucidate the causes of the much more complete depletion of the pelt brought about by the bacterial products of bates and puers. It has been pointed out (p. 82) that gelatine and hide-fibre in a neutral condition are swollen by water, but that the equilibrium so reached is an unstable one, easily influenced by slight causes. Among these, as has been pointed out by Koerner,[59] the surface-tension between the water and the swollen fibre holds a place; and surface-tensions of this sort are greatly influenced by many substances of the class to which bacterial ferments belong. Many salts also alter the water-absorption of gelatinous fibres, sometimes causing swelling, and sometimes contraction, according to temperature, concentration, and the nature of the salt. Though most salts do not seem to be absorbed by hide-fibre, it is possible, as suggested by Koerner (_loc. cit._), that in some instances the base may combine with the acid-groups, and the acid of the salt with the basic groups of the gelatine-molecule, while other cases are known in which salts are actually dissociated, and their acid fixed by the affinities of the hide-fibre. An interesting case of this sort was recently proved by Paessler and Appelius,[60] who showed that sulphuric acid was absorbed from a solution of hydric sodic sulphate, and the neutral sulphate left in the solution. Similar reactions undoubtedly occur with some salts of strong acids and weak bases, but this point must be more fully discussed in connection with the theory of mineral tannages. [59] Beiträge zur wissenschaftlichen Grundlage der Gerberei, Jahresberichte der deutschen Gerberschule zu Freiberg, 1898-9 and 1899-1900. [60] Wissenschaftliche Beilage des Ledermarkt, 1901, ii. p. 106. CHAPTER X. _WATER AS USED IN THE TANNERY._ Of all the materials employed in tanning, none is of more indispensable importance than water, and its quality has undoubtedly great influence on tanning, though it is constantly blamed for faults and troubles which are really due to the mistakes of the tanner.